EAGER: Collaborative Research: Developing new laser ablation (U-Th)/(He-Pb) hematite double dating techniques to date ancient oxidation

EAGER：合作研究：开发新的激光烧蚀 (U-Th)/(He-Pb) 赤铁矿双重测年技术来测定古代氧化的年代

• 批准号: 2203532
• 负责人: Rebecca Flowers
• 金额: $ 8.02万
• 依托单位: University of Colorado at Boulder
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2021
• 资助国家: 美国
• 起止时间: 2021-12-15 至 2023-11-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=2203532&HistoricalAwards=false
• 关键词: EAGER; Collaborative; Research; Developing; new

This project is focused on improving methods that Earth scientists can use to determine the formation age of the mineral hematite. Hematite is a wide-spread mineral in rocks and soils and often forms due to oxidation on Earth’s surface as well as on other planets — notably Mars. Hematite can incorporate uranium when it forms. Given that uranium undergoes radioactive decay it is possible to conduct both uranium-lead dating (U-Pb) and uranium-thorium-helium dating ((U-Th)/He) on hematite crystals. Investigators will apply methods that can make these measurements simultaneously at a small scale on hematite crystals by blasting them with a laser and measuring the isotopes through mass spectrometry. An additional property of hematite is that it records the magnetic field at the time it crystallizes. This ability to record the ancient magnetic field enables Earth scientists to reconstruct the past position of Earth’s continents and can also be used to gain insight into the timing of hematite formation. They will make magnetic measurements of hematite including measurements that use new capabilities to make magnetic maps at the microscopic scale using an instrument called a quantum diamond microscope. The study will focus on hematite within ancient sedimentary rocks known as iron formations. In the United States, iron formations in the Lake Superior region are the major source of domestic iron production. By applying these dating methods to iron formation, they will constrain the timing of hematite formation in these units, where the timing of oxidation (that is, hematite formation) is debated. If this project is successful, these combined methods of measuring the age of hematite will enable a multitude of future studies. For example, it would be possible to pursue more advanced studies on hematite formation within iron formation, to study the processes and timescales of deep soil leaching (also known as laterization), and determine the timing of ancient surface exposure and chemical alteration in far greater detail using these combined methods. The project will support the research of an early-career scientist, advance development of new analytical capabilities at both University of Carlifornia Berkeley and University of Colorado Boulder, and support both undergraduate and graduate student research for first-generation students.These investigators seek to develop simultaneous in situ laser ablation (U-Th)/He and U-Pb dating of hematite, which is termed LA-(U-Th)/(He-Pb). Although bulk (U-Th)/He and in situ (U-Pb) methods have been used previously on hematite, the coupled laser-ablation technique has never been applied. This method can provide a powerful tool for assessing the timing of oxidation and weathering in a wide range of hematite-bearing environments. Researchers will investigate the timing of hematite crystallization in Lake Superior region iron formation through this method development in conjunction with paleomagnetic data, which can provide complementary chronologic insight. They will focus on a carefully selected set of samples that will enable method development and give new insights into the origin of iron formations. All samples will be characterized prior to geochronologic measurements via electron backscatter diffraction and electron microprobe to understand the chemical heterogeneity of the samples and the distribution of crystallites within hematite aggregates. This characterization will permit targeting of individual crystallites through LA-(U-Th)/(He-Pb) and provide context relative to potential polycrystalline diffusion behavior. This study has two main sample targets: (1) large, high-purity hematite samples from iron formation that will be used for method development; (2) typical iron formation from the Menominee Group. Menominee Group samples will be analyzed through both LA-(U-Th)/(He-Pb) and paleomagnetism. Paleomagnetic analyses will be conducted at both the centimeter and micrometer scale to constrain hematite formation relative to folding and through comparison to Laurentia’s apparent polar wander path. The well-constrained regional history of deposition, tectonism, and near-surface weathering provides testable hypotheses for the timing of hematite formation in these samples. Successful radiometric dating of these materials will provide confidence in the utility of the LA-(U-Th)/(He-Pb) method in natural samples beyond museum-quality hematite specimens.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.

该项目的重点是改进地球科学家可以用来确定矿物赤铁矿形成年龄的方法。赤铁矿是一种广泛存在于岩石和土壤中的矿物，通常是由于地球表面以及其他行星（特别是火星）的氧化而形成的。赤铁矿在形成时可以与铀结合。由于铀经历放射性衰变，因此可以对赤铁矿晶体进行铀-铅定年（U-Pb）和铀-钍-氦定年（（U-Th）/He）。研究人员将应用可以同时在小规模上对赤铁矿晶体进行这些测量的方法，方法是用激光对它们进行爆破，并通过质谱法测量同位素。赤铁矿的另一个特性是它在结晶时记录磁场。这种记录古代磁场的能力使地球科学家能够重建地球大陆过去的位置，也可以用来深入了解赤铁矿形成的时间。他们将对赤铁矿进行磁性测量，包括使用一种称为量子金刚石显微镜的仪器在微观尺度上使用新功能制作磁性地图的测量。这项研究将集中在被称为铁地层的古代沉积岩中的赤铁矿。在美国，苏必利尔湖（Lake Superior）上级地区的铁矿是国内铁生产的主要来源。通过将这些测年方法应用于铁的形成，他们将限制这些单元中赤铁矿形成的时间，其中氧化（即赤铁矿形成）的时间是有争议的。如果该项目成功，这些测量赤铁矿年龄的组合方法将使未来的许多研究成为可能。例如，有可能对铁地层中的赤铁矿形成进行更深入的研究，研究深层土壤沥滤（也称为红土化）的过程和时间尺度，并使用这些组合方法更详细地确定古代表面暴露和化学蚀变的时间。该项目将支持早期职业科学家的研究，推动加州大学伯克利分校和科罗拉多大学博尔德分校新的分析能力的发展，并支持第一代学生的本科生和研究生研究。这些研究人员寻求开发同步的原位激光消融（U-Th）/He和U-Pb测年的赤铁矿，这被称为LA-（U-Th）/（He-Pb）。虽然块体（U-Th）/He和原位（U-Pb）的方法已被用于赤铁矿，耦合激光烧蚀技术从未被应用。该方法可以提供一个强有力的工具，评估氧化和风化的时间在广泛的含赤铁矿环境。研究人员将通过这种方法的开发，结合古地磁数据，研究苏必利尔湖上级地区铁矿形成的赤铁矿结晶时间，这可以提供补充的年代学见解。他们将专注于一组精心挑选的样品，这将有助于方法的发展，并对铁的形成提供新的见解。在地质年代学测量之前，将通过电子背散射衍射和电子微探针对所有样品进行表征，以了解样品的化学不均匀性和赤铁矿聚集体中微晶的分布。该表征将允许通过LA-（U-Th）/（He-Pb）靶向单个微晶，并提供相对于潜在多晶扩散行为的背景。这项研究有两个主要的样品目标：（1）将用于方法开发的大型高纯度铁矿石样品;（2）梅诺米尼组的典型铁矿石。Menominee组样品将通过LA-（U-Th）/（He-Pb）和古地磁进行分析。古地磁分析将在厘米和微米尺度上进行，以限制相对于褶皱的赤铁矿形成，并通过与Laurentia的明显的极移路径进行比较。沉积，构造作用和近地表风化的良好约束的区域历史为这些样品中赤铁矿形成的时间提供了可检验的假设。这些材料的成功放射性测年将为LA-（U-Th）/（He-Pb）方法在博物馆质量的赤铁矿样品之外的天然样品中的实用性提供信心。该奖项反映了NSF的法定使命，并被认为值得通过使用基金会的知识价值和更广泛的影响审查标准进行评估来支持。